As if you hadn't always wondered, here is the answer to your lifelong question. Here's an episode of Brainstuff, taken from the files of the classic and long-running HowStuffWorks podcast (the first ever) by Marshall Brain.
It turns out that 1 liter of helium can lift a gram of matter so all you have to do is calculate your weight in grams; one pound equals about 453.6 grams (of course those of you reading this outside the U.S. and Liberia know your weight in grams so you can skip this part). It also helps to know how many liters of helium the average balloon can hold - approximately 14. So, you take your weight in grams and divide that by 14 and you have the number of average sized balloons you'll need to lift you (don't forget to include the weight of the chair or whatever contraption you'll take your balloon ride in).
You will soon find getting your hands on that much helium may be tricky since the government's subsidization of industry that uses the element has artificially deflated (yes!) the market price for decades and led to a shortage. The balloons would cost you a few bucks too. You can get a bag of 100 for a mere $6.45 here, but you'll need several thousand of course, so your total at the checkout will certainly rise (again, yes!).
And it's at this point that I should say that if you are a precocious and financially liquid child with a big imagination, you should absolutely NOT actually try to lift yourself up with thousands of balloons. What sounds like a good idea would likely turn very bad very quickly. Instead, you can just use your imagination and math and pretend you're doing it, which, I realize now that I've written it, is how one describes fun as an adult.